Preparing Instructional Objectives by Robert F. Mager (1997) is the best book on the topic I have ever read. Not only does Mager provide excellent advice about writing instructional objectives, but he also makes what could possibly be a boring topic actually fun for the reader. The book reminds me of the adventure books I used to read as a child. In those books you would be given a scenario and a choice between multiple actions. Each action would have a page number to jump to in order to finish your adventure.
Mager uses a similar approach to allow for formative evaluation throughout this book. You test your knowledge, jump to the page of the answer you believe is correct. If you are correct, move on. If not, you will be directed to additional pages about the topic in which you are having trouble. Then, you can test your knowledge again to make sure you've got it. What a great idea!
The process made the book easy to read, not to mention efficient. I hope anyone who has to write objectives will pick up this book.
Mager, R.F. (1997). Preparing instructional objectives: A critical tool in the development of effective instruction (3rd Ed.). Atlanta, GA: The Center for Effective Performance, Inc.
You can find it on Amazon at http://www.amazon.com/Preparing-Instructional-Objectives-Development-Instruction/dp/1879618036/ref=sr_1_1?ie=UTF8&qid=1337218849&sr=8-1